1994 Suburban List of Problems. HELP!

I have a 1994 Suburban 1500 4X4 with 237,736 miles on it. As far as the engine goes it runs good and doesn't use any oil. The only thing is usually after about 5-10 minutes after it gets good and warm the service engine lignt will come on. After you shut the truck off it goes away untill it gets good and warm again the next time you run it. I think it is a sensor somewhere because it is running way to rich and getting bad gas mileage. Smells like it is burning raw gas at times. Another thing is the transmission. This is the BIG thing I need help with. It is really weird! When you start out driving and it doesn't matter weather you are going from park straight to drive or from park to reverse and then to drive, the trans vibrates. It isn't really a slip it is just a bad vibration. What I have been doing is going from park or reverse, depends on weather i am having to back up or not, straight to drive, get it moving forward at about 5mph and putting it into neutral for just a second and straight back into drive and somehow it fixes the vibration and it drives perfectly. It doesn't make a difference weather you go into over drive, drive, 2nd, or 1st you still have to throw it into neutral to fix the vibration. Any ideas?